1. Field of the Invention
The present invention relates to an injection molding method of metal material, and more particularly to an injection molding method of a metal material, by which nonferrous metal having a low melting point, such as zinc, magnesium, or alloy thereof, is completely melted to allow injection molding in a liquid phase state.
2. Detailed Description of the Prior Art
Attempts have been made to completely melt nonferrous metal having a low melting point so as to allow injection molding in a liquid phase state. Like in the case of injection molding of a plastic material, the molding method thereof adopts a heating cylinder having inside an injecting screw, which is allowed to rotate and move along the axial direction. A granular metal material supplied from the rear portion of the heating cylinder is heated and melted completely while being transferred toward the head of the heating cylinder by means of rotation of the screw, and after a quantity of the metal material in the liquid phase state is metered in the front chamber of the heating cylinder, the metal material is injected into a mold through the nozzle attached to the tip of the heating cylinder by moving the screw forward.
A problem occurring in case of adopting the foregoing injection molding for the metal material is that the material is neither transferred readily nor metered in a stable manner by means of rotation of the screw.
A molten plastic material has a high viscosity, and transfer of the molten plastic material by means of rotation of the screw is allowed mainly because a friction coefficient at the interface of the molten plastic material and the screw is smaller than a friction coefficient at the interface of the molten plastic material and the inner wall of the heating cylinder, and therefore, a difference in friction coefficient is produced between the two interfaces.
In contrast, the metal material, when melted completely to the liquid phase state, has such a low viscosity compared with the plastic material that a difference in friction coefficient is hardly produced between the above two interfaces. Hence, a transfer force such as the one produced with the molten plastic material by means of rotation of the screw is not readily produced.
However, a transfer force is produced with the metal material when it is in a solid state and in a high viscous region where the metal material is in a semi-molten state during the melting process. Thus, the metal material can be transferred by means of rotation of the screw up to that region. Nevertheless, as the metal material is further melted, the viscosity drops with increasing ratio of the liquid phase, and the transfer force produced by the screw groove between the screw flights decreases, thereby making it difficult to supply the molten metal material in a stable manner to the front chamber of the heating cylinder by means of rotation of the screw.
Because the molten plastic material has a high viscosity, it is stored in the front chamber of the heating cylinder by means of rotation of the screw, while at the same time, a material pressure that pushes the screw backward is produced as a reaction. By controlling the screw retraction caused by the material pressure, air bubbles or the like contained in the molten material can be deaerated, and further, the material can be metered in a stable manner.
However, the metal material in the low-viscous liquid phase state cannot produce a pressure high enough to push the screw backward. Thus, the screw retraction by the material pressure hardly occurs, and if the metal material is stored in the front chamber by means of rotation of the screw alone, a quantity thereof undesirably varies. This poses a problem that neither can steady (stable) metering nor satisfactory deaeration be readily conducted.
In addition, the metal material in the liquid phase state has a low viscosity and fluidity. For this reason, when it is allowed to stand by stopping rotation of the screw, the metal material in the liquid phase state goes into a clearance formed between the screw flights and heating cylinder, and heat is transferred to the screw when the metal material is brought into physical contact with the screw flights, thereby causing the metal material to be solidified. The resulting solid serves as resistance for the screw retraction, which may prevent a smooth operation of the screw.
The present invention is devised to solve the above problems raised with injection molding of a metal material in the liquid phase state, and therefore has an object to provide a novel injection molding method of a metal material, by which the metal material in the liquid phase state can be transferred, metered, and deaerated smoothly at all times by operating the movement and rotation of the screw.
In order to achieve the above and other objects, the present invention is an injection molding method of a metal material adapted to employ an injection apparatus comprising a heating cylinder provided with a nozzle at a tip thereof and a supply port at a rear portion thereof and having inside a screw, which is allowed to rotate and move along an axial direction, for injecting the metal material metered in a liquid phase state through the nozzle by moving the screw forward, and the method includes the following steps of: removing, after injection, retraction resistance made of the metal material having gone into a clearance between the heating cylinder and screw flights in advance by rotating the screw for a set number of times at a forward position; accumulating the metal material in the liquid phase state in a front chamber by forcing the screw to retract for a set distance, and applying a back pressure to the screw rotating at a retraction position, thereby starting transfer of the metal material; completing accumulation by stopping rotation of the screw; pressing the metal material accumulated by moving the screw forward; and effecting injection only when a material pressure reaches a set pressure within a preset forward distance of the screw, whereupon metering is assumed to have been completed.
The screw may include an injecting plunger at the tip, and the plunger has substantially a same diameter as a diameter of the front chamber formed in the heating cylinder at a top end portion by reducing a diameter thereof so as to be allowed to fit into the front chamber by moving forward and backward while securing a sliding clearance such that hardly causes a back flow of the metal material in the liquid phase state accumulated in the front chamber.
According to the present invention, the forced retraction of the screw is allowed smoothly by, after injection, rotating the screw at the forward position and removing in advance the resistance for the retraction of the screw made of the metal material having gone into the clearance between the heating cylinder and screw flights. Consequently, even if the metal material is in the low-viscous liquid phase state, the metal material can be transferred readily by the screw.
In addition, because the metal material in the liquid phase state can be stored primarily and the accumulation quantity of the metal material in the front chamber can be compensated by means of rotation of the screw, a shortage in the accumulation quantity before metering can be prevented. Further, deaeration in the front chamber can be conducted by moving the screw forward before injection. In addition, injection is effected only when it is confirmed that at least a predetermined quantity has been metered based on the forward distance and material pressure, injection molding of a product made of the metal material in a stable injection state can be performed.